Help with 2D AI scripting

This script should work as the player enter the range of enemy then the enemy will charge/move towards the player ... but the enemy are sit still and not moving .. can anybody fix or point out my script ?

 public float enemySpeed;


 //facing
 public GameObject enemyGraphic;
 bool canFlip = true;
 bool facingRight = false;
 float flipTime = 5f;
 float nextFlipChance = 0f;



 //attacking
 public float chargeTime;
 float startChargeTime;
 bool charging;
 Rigidbody2D enemyRB;

 void Start (){
     enemyRB = GetComponent<Rigidbody2D> ();
 }

 
 // Update is called once per frame
 void Update () {
     if (Time.time > nextFlipChance) {
         if (Random.Range (0, 10) >= 5) flipFacing();
         nextFlipChance = Time.time + flipTime;

     }

 
 }

 void OnTriggerEnter2D(Collider2D other)
 {
     if (other.tag == "Player")
     {
         if (facingRight && other.transform.position.x < transform.position.x)
         {
             flipFacing();
         }
         else if (!facingRight && other.transform.position.x > transform.position.x)
         {
             flipFacing();
         }
         canFlip = false;
         charging = true;
         startChargeTime = Time.time + chargeTime;

     }

 }

 void OnTriggerStay2D(Collider2D other)
 {
     if (other.tag == "Player")
     {
         if (startChargeTime < Time.time)
         {
             if (!facingRight)
                 enemyRB.AddForce(new Vector2(-1, 0) * enemySpeed);
             else enemyRB.AddForce(new Vector2(1, 0) * enemySpeed);

         }
     }
 }

 void OnTriggerExit2D(Collider2D other)
 {
     if (other.tag == "Player")
     {
         canFlip = true;
         charging = false;
         enemyRB.velocity = new Vector2(0f, 0f);
     }
 }

 void flipFacing(){
     if (!canFlip) return;
     float facingX = enemyGraphic.transform.localScale.x;
     facingX *= -1f;
     enemyGraphic.transform.localScale = new Vector3(facingX, enemyGraphic.transform.localScale.y, enemyGraphic.transform.localScale.z);
     facingRight = !facingRight;

 }